Only $2 \mathrm{~mL}$ of $\mathrm{KMnO}_4$ solution of unknown molarity is required to reach the end point of a titration of $20 \mathrm{~mL}$ of oxalic acid $(2 \mathrm{M})$ in acidic medium. The molarity of $\mathrm{KMnO}_4$ solution should be ________ M.